A laser temperature-jump apparatus for the study of fast reactions in solution

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Abstract
An apparatus for the determination of the rates of fast chemical reactions in solution is described. The temperature of a solution is raised by a pulse of radiation from a laser, and the resulting shift of chemical equilibrium is followed spectrophoto metrically. If the laser is used in the non-Q-switched mode, and the optical energy is converted to thermal energy by a dye, temperature rises of 5K in 300 mu s are readily obtained, giving signal-to-noise ratios of over 100. If the laser is used in the Q-switched mode, so that the light pulse duration is of the order of 1 mu s, dye conversion is inefficient, due to the finite lifetime of the excited states of the dye, and temperature rises of less than 1 K are obtained. Direct conversion of optical energy to thermal energy by solvent absorption is also inefficient, since the absorbance of the usual solvents does not have a suitable value at any of the wavelengths of the light generated by the widely used lasers (694 nm and 1060 nm). Shock-wave transients, caused by high rates of energy input to the solvent, also limit the temperature-jump which can be usefully produced.